(1) Field of the Invention
The present invention relates to a filter apparatus for use in a radio apparatus for multiplex radio communication used when a signal is branched, a jig for arranging dielectrics of the filter apparatus for use in a radio apparatus, and a method for arranging dielectrics of the filter apparatus for use in a radio apparatus using the jig.
(2) Description of Related Art
As shown in FIG. 18, a multiplex radio apparatus 100 for multiplex radio communication has, in general, a high-frequency transmitting unit 110 for converting signals into RF signals as high-frequency input signals suitable for being transmitted by place propagation and transmitting the signals, a high-frequency receiving unit 120 for receiving the RF signals transmitted by the high-frequency transmitting unit 110 and demodulating the signals, a transmitting-receiving multiplexer 129, and an antenna 130.
The high-frequency transmitting unit 110 transmits signals from plural transmitting systems (FIG. 18 shows only a first transmitting system 110A and a second transmitting system 110B). The first transmitting system 110A has a modulating unit (MOD) 111A, an up-converter (U/C) 117A, and an amplifier 114A. The second transmitting system 110B has a modulating unit (MOD) 111B, an up-converter (U/C) 117B, and an amplifier 114B.
The up-converter 117A has a mixing circuit 112A and a voltage-controlled oscillator (VCO) 113A. The up-converter 117B has a mixing circuit 112B and a voltage-controlled oscillator (VCO) 113B.
The first transmitting system 110A and the second transmitting system 110B are connected to circulators (CIRs) 116A and 116B, respectively, via a filter bank 115.
The filter bank 115 removes unnecessary wave components in the RF signals inputted from the amplifiers 114A and 114B. The circulators 116A and 116B each have several terminals (two or three terminals in FIG. 18), transmitting an RF signal received through a certain terminal to an adjacent terminal in a specific direction to prevent a reverse travel of the signal in the transmission.
In the high-frequency receiving unit 120, any one of plural receiving systems (FIG. 18 shows only a first receiving system 120A and a second receiving system 120B) receives a signal according to a frequency characteristic of the received signal. The first receiving system 120A has a demodulating unit (DEM) 121A, a down-converter (D/C) 127A, a low noise amplifier (LNA) 124A. The second receiving system 120B has a demodulating unit (DEM) 121B, a down-converter (D/C) 127B, a low noise amplifier (LNA) 124B.
The down-converter 127A has a mixing circuit (a frequency converting circuit) 122A and a voltage-controlled oscillator (VCO) 123A. The down-converter 127B has a mixing circuit 122B and a voltage-controlled oscillator (VCO) 123B.
The first receiving system 120A and the second receiving system 120B are connected to circulators (CIRs) 126A and 126B, respectively, via a filter bank 125.
The circulators 126A and 126B each have several terminals (two or three terminals in FIG. 18) similar to the circulators 116A and 116B, transmitting an RF signal received through a certain terminal to an adjacent terminal in a specified direction. The RF signal as a received signal inputted through the antenna 130 and the transmitting-receiving multiplexer 129 is transmitted toward the circulator 126B from the circulator 126A.
The filter bank 125 removes unnecessary wave components of the RF signals as received signals inputted through the antenna 130 and the transmitting-receiving multiplexer 129. The filter bank 125, at the same time, branches the received signal according to a frequency characteristic of the signal and transmits the signal to either the first receiving system 120A or the second receiving system 120B.
In the multiplex radio apparatus 100 with the above structure shown in FIG. 18, an RF signal is transmitted through the high-frequency transmitting unit 110, the transmitting-receiving multiplexer 129 and the antenna 130 when the signal is transmitted. On the other hand, the RF signal is received through the antenna 130 and the transmitting-receiving multiplexer 129 in the multiplex radio apparatus on the receiver side. The received RF signal is branched in the filter bank 125 according to a frequency characteristic of the signal, and transmitted to either the first receiving system 120A or the second receiving system 120B.
In practice, the high-frequency receiving unit 120 has, as shown in FIG. 19, a branching filter 131 for receiving signals which has plural circulators (CIRs) 132 through 136 and the filter bank 125 including plural dielectric filters 137 through 141. Incidentally, FIG. 20 shows the circulators 132 and 133, and the dielectric filters 137 and 138 shown in FIG. 19.
Namely, the branching filter 131 used in the mutliplex radio apparatus has n circulators and n dielectric filters in the case of n branches. Incidentally, a TE01.delta. mode dielectric filter, which has a small loss of a high-frequency signal, is used as the dielectric filter.
In the high-frequency receiving unit 120 shown in FIG. 19, a microwave signal (which has various frequency components f.sub.1 through f.sub.n) as a received signal (an RF signal) received by the antenna 130 is inputted from a port 0 to the branching filter 131.
The inputted microwave signal is inputted to the dielectric filter 137 having a pass frequency band of a frequency f.sub.1 via the circulator 132. The microwave signal having a frequency f.sub.1 then passes through the dielectric filter 137, and is outputted to a port 1. On the other hand, the microwave signal having frequencies f.sub.2 through f.sub.n is reflected by the dielectric filter 137, then inputted to the dielectric filter 138 having a pass frequency band of a frequency f.sub.2 via the circulators 132 and 133.
In the dielectric filter 138, only the microwave signal having a frequency f.sub.2 is allowed to pass through the dielectric filter 138 and outputted to a port 2. The remaining microwave signal having frequencies f.sub.3 through f.sub.n is reflected by the dielectric filter 138, and inputted to the dielectric filter 139 having a pass frequency band of a frequency f.sub.3 via the circulators 133 and 134.
Likewise, in the dielectric filter 139, only the microwave signal having a frequency f.sub.3 is allowed to pass through the dielectric filter 139, and outputted to a port 3. In the dielectric filter 140 having a pass frequency band of a frequency f.sub.4, only the microwave signal having a frequency f.sub.4 is allowed to pass through the dielectric filter 140, and outputted to a port 4. This process is repeated. In the dielectric filter 141 having a pass frequency band of a frequency f.sub.n, only the microwave signal having a frequency f.sub.n is allowed to pass through the dielectric filter 141, and outputted to a port n.
As above, the branching filter 131 branches the microwave signal to the ports 1 through n by means of the dielectric filters 137 through 141 having the respective pass frequency bands of f.sub.1 through f.sub.n. In other words, if a pass frequency of a filter of the ith port is f.sub.i, a microwave signal having a frequency f.sub.i appears at this port.
Incidentally, the branching filter 131 is grounded via a terminal resistor 142.
Since the general branching filter 131 as above uses circulators and filters as many as branches, the general branching filter 131 has drawbacks that a space for mounting the branching filter 131 increases and it is difficult to reduce a cost of the branching filter 131.
When the microwave signal inputted through the port 0 is outputted to the port 2 after inputted to the dielectric filter 137, the microwave signal passes the circulator 132 twice, as shown in FIG. 21. Namely, the microwave signal branched to the ith port passes through the circulator 132 (2i -1) times.
However, a loss generated when the microwave signal passes the circulator once is from 0.1 to 0.2 dB. If the number of the circulators increases, the number of times the microwave signal passes through the circulators also increases. In consequence, the general branching filter has another drawback that a loss generated when the microwave passes through the circulators increases.
With an increase of a loss generated when the microwave signal passes through the circulators, noise components in the microwave signals increases, as well. For this, a ratio of signal components to noise components (an S/N ratio) of the branched microwave signal increases, which leads to a degradation of accuracy in the communication.